I'm trying to install this add-on: blender_mmd_tools
Installable.

1) Unzip the blender_mmd_tools-master.zip file you downloaded

2) Open the folder containing your Blender.app

3) Right-click Blender.app and select "Show Package Contents"

4) Then open the "Contents" folder, followed by "Resources", then "2.76" (or whatever version of Blender you may have), then "scripts", and finally open the "addons" folder

5) Now inside the "addons" folder, you want to copy and paste the "mmd_tools" folder that was unzipped from inside the blender_mmd_tools-master.zip file

6) Now open (or restart) the Blender app and go to the User Preferences and search mmd_tools and check to use the add-on and "Save User Settings"

7) Now go back to 3D view and it should be a new tab called mmd_tools

Have fun!